1. Field
The present invention relates to a motor control device driving a permanent magnet motor in which a rotor includes permanent magnets each having such a low coercive force that an amount of magnetization is changeable, a motor drive system comprising the permanent magnet motor and the motor control device, a washing machine comprising the motor drive system, an air conditioner comprising the motor drive system, and a method of changing a magnetization amount of the permanent magnet motor.
2. Related Art
Techniques for variable speed operation of permanent magnet motors ranging from low to high speeds have recently become widespread for the purpose of reducing electric power consumption. A vector control is executed for the permanent magnet motors by means of an inverter device in the techniques. Japanese patent application publication JP-A-2006-280195 (hereinafter, “patent document 1”) discloses a permanent magnet motor which can improve a motor efficiency over an entire variable speed range and realize an improvement in reliability. The disclosed permanent magnet motor is configured as follows. The permanent magnet motor comprises a stator provided with windings, permanent magnets each having such a low coercive force that a flux density is irreversibly changed by a magnetic field established by stator winding current, and a rotor provided with high coercive permanent magnets each having a coercive force twice as large as the low coercive force or above. When the motor is driven at a high speed that requires voltage equal to or larger than a maximum power-supply voltage, a total interlinkage magnetic flux is adjusted by magnetizing the low coercive permanent magnets by a magnetic flux due to current so that the total interlinkage magnetic flux by the low and high coercive permanent magnets is reduced.
In patent document 1, stator windings of the motor are energized by d-axis current obtained by a vector control when low coercive permanent magnets are magnetized. A current value and the number of times of energization are determined based on experimental data. For example, the current value and number of times of energization are determined based on the experimental result that the motor flux content is reduced by 10% when 10 amperes of d-axis current are applied in the negative direction. In this case, however, an actual rotor flux content of the motor is uncertain after the low coercive permanent magnets are magnetized or demagnetized. Thus, there is a problem that the motor cannot always be operated with an optimum efficiency.